Question

A reaction has a rate law of Rate = (1.25 M⁻²s⁻¹)[A][B]². What concentration of [B] would give the reaction a rate of 0.0857 M⁻¹s⁻¹ if the concentration of [A] is 0.250 M?

Answer 1

Given

Rate expression of the reaction = 1.25 [A] [B]².......{I}

It implies that :-

• Reaction is 1st order with respect to A.

• Reaction is second order with respect to B.

To find

Concentration of [B] when the rate is already given.

Calculation

Given, at any time t

R = 0.0857

[A] = 0.250

Putting these values in eqn {I}

=> 0.0857 = 1.25 [A] [B]²

=> 0.0857 = 1.25 × 0.250 × [B]² ( [A] = 0.250)

=> 0.0857 = 0.3125 [B]²

=>[B]² = 0.27424

=>[B] = 0.523

Hence, the concentration of B at that time will be 0.523 M.